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Tropical Storm Marie Forecast Discussion Number 4
2020-09-30 04:35:27| National Hurricane Center (East Pacific)
Issued at 900 PM MDT Tue Sep 29 2020 000 WTPZ43 KNHC 300235 TCDEP3 Tropical Storm Marie Discussion Number 4 NWS National Hurricane Center Miami FL EP182020 900 PM MDT Tue Sep 29 2020 Infrared and passive microwave imagery indicate that Marie has continued to become better organized despite moderate northeasterly vertical wind shear of about 15 kt. The deep convection is now primarily confined to a single tightly curved band that wraps around the west side of the circulation. A late-arriving 29/1718Z ASCAT-C scatterometer pass revealed that Marie has a very tight inner-core circulation with a radius of maximum winds (RMW) of only 10-15 nmi and a small 34-kt wind radii extending outward to only about 20 nmi The scatterometer data also showed some 34-38 kt surface wind vectors in all quadrants. A blend of the scatterometer wind data and subjective Dvorak intensity estimates of of T2.5/35 kt and T3.0/45 kt from SAB and TAFB, respectively, the intensity has been increased to 40 kt. The initial motion estimate is 275/11 kt. As mentioned in the previous discussion, Marie's forecast track remains fairly straightforward. The cyclone is expected to move generally westward to west-northwestward for the next few days as it skirts the southern periphery of a deep-layer ridge to the north. Thereafter, a slower motion toward the northwest is forecast as Marie moves into a weakness in the ridge. The new NHC track forecast is similar to but a little to the left of the previous advisory track, and lies down the middle of the tightly packed consensus track model guidance envelope. Although the GFS- and ECMWF-based SHIPS intensity models have backed off significantly on predicting rapid intensification (RI) on this cycle, several environmental and cyclone structural parameters are still favorable for RI to occur. For about the next 72 hours, sea-surface temperatures (SST) are forecast to remain above 28C, alone with mid-level humidity values above 70 prevent and low wind shear values around 5 kt. Those environmental conditions coupled with Marie's small RMW should result in RI beginning in about 24 hours and continuing until the 60-h time period. As result, Marie is forecast to strengthen into a hurricane in about 24 hours and possibly become a major hurricane in 60-72 hours. Rapid weakening is expected on days 4 and 5 due to SSTs less than 26C, likely significant cold upwelling owing to Marie's expected large and strong wind field, and increasing southwesterly vertical shear of 20-25 kt. The NHC official intensity forecast is similar to the previous advisory, and closely follows a blend of the Decay-SHIPS and LGEM statistical models and the corrected-consensus HCCA model. Marie's 12-ft sea radii remain larger than would be expected for a small tropical storm due to a large fetch of southerly swell originating from the Southern Hemisphere. FORECAST POSITIONS AND MAX WINDS INIT 30/0300Z 13.8N 110.4W 40 KT 45 MPH 12H 30/1200Z 14.1N 112.9W 50 KT 60 MPH 24H 01/0000Z 14.6N 115.9W 60 KT 70 MPH 36H 01/1200Z 15.0N 118.8W 75 KT 85 MPH 48H 02/0000Z 15.5N 121.4W 90 KT 105 MPH 60H 02/1200Z 16.5N 123.8W 105 KT 120 MPH 72H 03/0000Z 17.6N 125.9W 100 KT 115 MPH 96H 04/0000Z 19.8N 129.3W 80 KT 90 MPH 120H 05/0000Z 21.4N 132.1W 60 KT 70 MPH $$ Forecaster Stewart
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